diff --git a/drivers/net/sfc/net_driver.h b/drivers/net/sfc/net_driver.h
index 40c0d931b1824b36c81a030f1f3ada13c3664fc5..6b2e4402ec576e6163b35d2cd9509ebbf77317c0 100644
--- a/drivers/net/sfc/net_driver.h
+++ b/drivers/net/sfc/net_driver.h
@@ -649,6 +649,7 @@ union efx_multicast_hash {
  * struct efx_nic - an Efx NIC
  * @name: Device name (net device name or bus id before net device registered)
  * @pci_dev: The PCI device
+ * @port_num: Index of this host port within the controller
  * @type: Controller type attributes
  * @legacy_irq: IRQ number
  * @workqueue: Workqueue for port reconfigures and the HW monitor.
@@ -732,6 +733,7 @@ union efx_multicast_hash {
 struct efx_nic {
 	char name[IFNAMSIZ];
 	struct pci_dev *pci_dev;
+	unsigned port_num;
 	const struct efx_nic_type *type;
 	int legacy_irq;
 	struct workqueue_struct *workqueue;
@@ -834,7 +836,7 @@ static inline const char *efx_dev_name(struct efx_nic *efx)
 
 static inline unsigned int efx_port_num(struct efx_nic *efx)
 {
-	return PCI_FUNC(efx->pci_dev->devfn);
+	return efx->port_num;
 }
 
 /**
diff --git a/drivers/net/sfc/siena.c b/drivers/net/sfc/siena.c
index 727b4228e0819bfe336e38ed6504bd8c2d852edc..7ecd255a7cc0075ccfb4c2c0a852e1696e9e6dcf 100644
--- a/drivers/net/sfc/siena.c
+++ b/drivers/net/sfc/siena.c
@@ -206,6 +206,7 @@ static int siena_probe_nic(struct efx_nic *efx)
 {
 	struct siena_nic_data *nic_data;
 	bool already_attached = 0;
+	efx_oword_t reg;
 	int rc;
 
 	/* Allocate storage for hardware specific data */
@@ -220,6 +221,9 @@ static int siena_probe_nic(struct efx_nic *efx)
 		goto fail1;
 	}
 
+	efx_reado(efx, &reg, FR_AZ_CS_DEBUG);
+	efx->port_num = EFX_OWORD_FIELD(reg, FRF_CZ_CS_PORT_NUM) - 1;
+
 	efx_mcdi_init(efx);
 
 	/* Recover from a failed assertion before probing */